Lithium ion secondary batteries (to also be referred to as “lithium ion batteries”) are being actively developed for consumer applications used in portable terminal devices such as cell phones as well as video cameras and the like due to their high energy output despite their extremely thin shape and compact size. Metal enclosure types have conventionally been used for the outer cover materials of lithium ion batteries. However, deep-drawn formed products obtained by cold forming a laminated film having a multilayer configuration (employing, for example, a configuration consisting of a base material layer having heat resistance, an aluminum foil layer and a sealant (heat-fusible film) layer) (to also be simply referred to as “deep-drawn formed products”) have come to be used in recent years due to their advantages of being lightweight and allowing greater freedom in the selection of battery shape. In addition to offering the aforementioned advantages, these deep-drawn formed products are also advantageous in terms of having high heat dissipation and low cost, and studies are being conducted on their application to the batteries of environmentally-friendly hybrid vehicles and electric vehicles.
A lithium ion battery that uses the aforementioned laminated film is formed by housing battery body components in the form of a positive electrode material, a negative electrode material and a separator in a deep-drawn formed product together with an electrolytic solution, obtained by dissolving a lithium salt in an aprotic solvent such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate or ethylmethyl carbonate, or an electrolyte layer composed of a polymer gel impregnated with that electrolytic solution, followed by encapsulating by heat sealing.
The aforementioned electrolytic solution is highly permeable with respect to sealant layers composed of a heat-fusible film. If the electrolytic solution penetrates into the sealant layer, the penetrated electrolytic solution lowers lamination strength between the aluminum foil layer and the sealant layer and the electrolytic solution may ultimately leak to the outside. In addition, examples of lithium salts serving as electrolytes include lithium salts such as LiPF6 and LiBF4. However, if moisture penetrates into a deep-drawn formed product, the lithium salt ends up being hydrolyzed resulting in the formation of hydrofluoric acid, thereby causing corrosion of metal surfaces and a decrease in lamination strength between each layer of the multilayer film.
In this manner, an outer cover material for a lithium ion battery having a multilayer configuration in the manner of a laminated film is required to inhibit corrosion of metal foil (aluminum foil) and a decrease in lamination strength between each layer caused by the electrolytic solution. In addition, the outer cover material is also required to be resistant to electrolytic solution and hydrofluoric acid.
Chromate treatment, that uses hexavalent chromium and is carried out on the surface of the aluminum foil layer, has been conventionally used as a method for enhancing adhesion between the aluminum foil layer and a base material layer in outer cover materials for lithium ion batteries. However, hexavalent chromium has been treated as an environmentally hazardous substance in recent years as in the Rohs and REACH regulations in Europe. Consequently, trivalent chromium has come to be used in chromate treatment. However, in this method, a hexavalent chromium treatment layer is formed by using trivalent chromium as a starting substance. Since there is the possibility of the complete discontinuation of the use of chromium in the future, and in consideration of applications to electric vehicles designed on the basis of environmental considerations in particular, it is important to develop a method that enhances corrosion prevention performance against electrolytic solutions and hydrofluoric acid with a treatment that does not use chromium compounds.
On the other hand, the size of lithium ion batteries can be reduced due to their high energy density. The magnitude of the energy density of a lithium ion battery is determined by the extent to which cells and electrolytic solution can be contained in a single battery, and the contained amounts thereof are determined by the forming depth when forming an outer cover material for a lithium ion battery and obtaining a deep-drawn formed product. Although drawing is typically carried out with a metal mold, if the forming depth is too deep, cracks and pinholes may form in those portions of the lithium battery outer cover material that are drawn by forming, thereby resulting in a loss of reliability of the battery. Consequently, in order to realize both battery high reliability and high energy density, lithium battery outer cover materials are required to have superior deep-drawing formability. In the case of applying a lithium ion battery to an electrical vehicle and the like in particular, although there is a desire to extract a large amount of current, since it is also desired to obtain superior long-term storage stability, further improvement of deep-drawing formability is required.
The following have been indicated as outer cover materials having enhanced deep-drawing formability.
(i) An outer cover material that uses for the base material layer thereof a drawn film having specific tensile strength and elasticity in the four directions of 0° C., 45°, 90° and 135° relative to the direction of drawing, and having little directivity of mechanical properties (Patent Document 1).
(ii) An outer cover material that uses for the base material layer thereof a heat-resistant resin film having impact strength of 30000 J/m or more (Patent Document 2).
(iii) An outer cover material that uses for the base material layer thereof a biaxially drawn polyamide film having density of 1142 kg/cm3 to 1146 kg/cm3 (Patent Document 3).
(iv) An outer cover material that uses as a base material layer thereof a heat-resistant resin drawn film having a shrinkage factor of 2% to 20% (Patent Document 4).
In addition, outer cover materials are also required to have superior moldability. In other words, since energy density is determined by the extent to which cells and electrolytic solution can be housed in a lithium battery, in order to further increase the housed amounts thereof, it is necessary to be able to increase the molding depth when molding the outer cover material into the shape of the battery.
Although molding of outer cover materials is typically carried out by drawing using a metal mold, if the molding depth at this time is excessively deep, cracks and pinholes may form in those portions that are drawn by molding, thereby resulting in a loss of battery reliability. Consequently, it is important to be able to increase molding depth without losing reliability.
In large-scale applications such as electric vehicles in particular, although there is a desire to further increase energy density in terms of battery performance with respect to the desire to extract a large current, superior reliability and long-term storage stability in particular are also simultaneously required.
Various outer cover materials are known that demonstrate improved moldability, examples of which include those using for the base material layer thereof a biaxially drawn polyamide film having superior kinetic coefficient of friction for the surface of the base material layer, rupture strength, elasticity to rupture point, shrinkage factor, impact strength, density or refractive index (Patent Documents 2, 4 and 5 to 10). However, further improvement of outer cover material moldability is required in large-scale applications in particular.